Lay the new elastic flat on a table. Use the old elastic as a reference, measure the same length with a measuring tape, and mark it with tailor's chalk.

If your old elastic was overly damaged, shorten the length by 1-2 inches to ensure tightness.

With sewing scissors, cut the new elastic at the marked length.

If you are unsure of the length, you can start with a longer piece and trim the excess afterwards.